The present invention relates to systems which use adaptive equalizers.
In voiceband and other data transmission systems, a so-called secondary channel signal, which carries diagnostics, maintenance, and other information, is often frequency-division-multiplexed with the primary channel signal which carries the users' data. Conventionally, bandpass filters have been used in the receiver portion of voiceband modems to remove, for example, the secondary channel energy from the received signal in order to equalize and otherwise process the primary channel and recover the data carried therein.
In general, this approach is satisfactory. However, as voiceband systems are designed with higher and higher bit rates, the requirements on the filter used to remove the secondary channel energy get more and more severe. This is because it becomes increasingly important for the primary channel signal to be passed undistorted through the filter. For example, in a voiceband data communication system transmitting data at 19.2 Kbps, it has been found that a 28th order filter is needed. Such a filter adds not insignificantly to the cost, power dissipation and circuit board area requirements of the modem.